How close is too close?

Well it all depends on how much recoil the cartridge has. Under recoil the scope objective can flex . With big recoil and long scopes a lot more than 1/16 is possible .
Make sure the front ring is as close to the objective taper as possible to reduce overhang at the front. Keep an eye on the scope surface to see if any contact mark shows up. If not no big problem .
In a low recoil gun I can't see a flexing issue .